Radar sensors are used in motor vehicles to acquire the surrounding traffic, and supply location data such as distances, relative speeds, and directional angles of objects in the environment surrounding the vehicle. These location data are used as a basis for various driver assistance functions, such as automatic distance regulation, a collision warning system, or a collision avoidance system. The location data of such radar sensors are also required in systems for autonomous driving.
In disadvantageous conditions, for example when a coating of slush forms on the radome of the radar sensor during heavy snowfall, the radar signals can be attenuated so strongly that in practical terms the radar sensor is blinded. Because the functions based on the location data of the radar sensor are relevant for safety, the radar sensor should be closely monitored for possible blindness during operation.
German Patent Application No. DE 199 45 268 A1 and PCT Application. No. WO 01/22113 A1 describe methods in which the average power of the signals received by the radar sensor is used as an indicator for radar sensor blindness recognition. This indicator is based on the idea that as the blindness of the sensor increases, the power level it can accept from the outside will decrease. In an FMCW radar unit, this indicator can be improved by varying the transmit power during the transmission of at least two successive, identical frequency ramps. Here, the identical ramps are traversed one after the other with different transmit power levels, and the power levels received by the sensor and averaged are compared to one another.
However, the indicators enable only a quasi-binary judgement to be made as to whether the sensor is absorptively coated or not. As the demands made by assistance functions on sensor coating recognition increase (especially in systems for autonomous driving), there is increasing interest in finding an answer to the question how strongly the sensor is coated, and whether the requested function can still be carried out given the existing coating.